1. Field of the Invention
This invention relates to the art of gas pressure measurement in the pressure range of 10.sup.-.sup.1 to 10.sup.-.sup.7 torr by cold cathode ionization gauges. The invention is directed specifically to the circuit which controls such gauges and similar types of loads.
2. Description of the Prior Art
A cold cathode ionization gauge includes a gauge tube and a controller circuit. One typical cold cathode ionization gauge tube consists of a central cathode wire held at a potential of the order of kilovolts with respect to a coaxially surrounding anode within a magnetic field of the order of kilogauss oriented parallel to the cathode. When a suitably intense electric field is applied across the cathode and anode, electrons are derived from the cathode by field emission. These electrons are constrained by the magnetic field to trajectories about the central cathode. By collisions of these electrons with residual gas molecules present in the volume of the gauge, positive ions and additional free electrons are generated. The additional electrons, picking up kinetic energy from the radial electric field and confined by the axially directed magnetic field, are in turn precursors of subsequent ionizing collisions. The avalanche of ionization reaches an equilibrium value limited by the numerical density of gas molecules within a time of the order of 10.sup.-.sup.3 seconds depending upon the pressure. The positive ions, which are affected only slightly by the magnetic field, are collected at the cathode and result in a net current, functionally related to the pressure, ultimately drawn from the power supply.
The conventional cold cathode gauge employs a d.c. potential across the gauge tube. The current across gauge tubes increases with increase in pressure. Thus, if the d.c. potential is high enough for gas discharge at low pressure such as 10.sup.-.sup.6 torr, the power dissipated at high pressures, such as 10.sup.-.sup.2 torr, becomes too great to be sustained without damage to the gauge tube. The present invention overcomes the problem by providing substantially constant full peak potential at the low pressure extreme and providing a lower potential (average potential) at high pressures.
A different cold cathode gauge of the prior art, U.S. Pat. No. 3,447,072, was designed to excite the gauge tube with an alternating potential. In this apparatus the gauge tube is subject to the full cycle of an a.c. waveform in contrast to the present invention wherein only that half cycle is employed which causes the ion current to be collected at the cathode, namely the negative half cycle. This gauge is further distinguished by the invariance of the waveform as contrasted with the present invention in which the shape of the waveform varies over the pressure range of the instrument.